1. Field of the Invention
The present invention relates to a novel optical recording material having high sensitivity, an optical recording medium employing the same, and recording and reproducing methods therefor.
2. Description of the Background Art
In recent years, study has been widely made on an optical recording medium employing a photochromic compound. Upon irradiation with light of a prescribed wavelength, such a photochromic compound is changed in molecular structure by photochemical reaction to cause changes in optical properties such as absorbance, angle of rotation, refractive index and the like. Through such differences caused in the optical properties, it is possible to record and reproduce information in and from the optical recording medium by irradiating the same with light of a specific wavelength, while it is also possible to erase the information by converting the molecular structure of the photochromic compound to the original state again.
In order to apply such a photochromic compound to an optical recording medium, it is necessary to satisfy the following conditions (1) to (5):
(1) thermal stability (stability in preservation of recorded information) PA1 (2) durability against repetition (recording-erasing reloadable frequency) PA1 (3) semiconductor laser sensitivity PA1 (4) high sensitivity (large changes of optical properties in a thin Film state) PA1 (5) durability against repetitive reproduction (reproducible frequency)
Deep study has been made on the condition (1) of thermal stability, which has generally been regarded as the most important subject. A diarylethene derivative having a furan ring and a thiophene ring has been reported as a substance which substantially satisfies practical properties. Also as to the condition (2) of durability against repetition, it has been reported that a diarylethene derivative having a benzothiophene ring or an indole ring can be repeatedly used up to about 10.sup.4 times under the degassing condition.
Deep study has also been made on the conditions (3) and (5) of semiconductor laser sensitivity and durability against repetitive reproduction, and great effects have been attained in relation to a diarylethene derivative. For example, Bull. Chem. Soc. Jpn. by Uchida, Nakayama and Irie, 63, 1311-1315 (1990) discloses such a photochromic compound of 2,3-bis-(2-methylbenzo[b]thiophene-3-yl) maleic anhydride which is expressed in the following formula: ##STR2##
Methods of synthesizing such a photochromic compound are described in Japanese Patent Laying-Open Nos. 63-24245 (1988) and 3-75635 (1991), for example.
However, such a conventional photochromic compound is still insufficient in practice due to a small change of the absorption spectrum in a thin film state.
In application to an optical recording medium, such a photochromic compound must be independently employed or dispersed in a polymer for forming a recording layer. In this case, it is necessary to reduce the thickness of such a recording layer to the minimum, in order to condense a laser beam to a spot diameter of about 1 .mu.m. If the thickness of the recording layer is insufficiently reduced, the spot diameter of the laser beam is disadvantageously spread in the direction of depth, to cause reduction in recording density.
Therefore, it is necessary to maximize changes of the optical properties of the photochromic compound itself, so that sufficient changes of optical properties can be attained also when the recording layer is reduced in thickness. In recording/reproduction of information, an excellent signal can be obtained as the absolute values of such changes are increased.
It is known that, when the photochromic compound of the aforementioned diarylethene derivative is in a solid state being dispersed in a polymer, its reactivity is reduced as compared with that in a liquid state. It is also known that such a photochromic compound is reduced in reactivity when its concentration is increased in the polymer (Preliminary Reports for the 61st Spring Conference, the Chemical Society of Japan, II2G535, p. 2114). Thus, awaited is a photochromic compound having large absolute values of optical property changes.
The aforementioned photochromic material enters a photostationary state containing molecules of both ring-opening and ring-closure states to be colored red upon irradiation with light of a wavelength near 430 nm, for example, while the same enters a complete ring-opening state upon irradiation with light of a wavelength near 550 nm.
Therefore, it is possible to apply one of such reversibly changed states to a recorded state and the other one to an erased state. Further, it is possible to read information as recorded by irradiating the photochromic material with light of a specific wavelength (550 nm, for example) and detecting differences caused in optical properties such as absorbance between the two states.
However, when the photostationary state is applied to a recorded state and the ring-opening state is applied to an erased state in an optical recording medium containing such a photochromic material, for example, the photochromic material enters a ring-opening state upon irradiation with light of a wavelength near 550 nm for reproduction, to destroy information as recorded. If the optical recording medium is alternatively irradiated with light of a wavelength near 430 nm for reproduction, a portion of an unrecorded state (erased state) disadvantageously enters a photostationary state (recorded state) to cause erroneous recording and destroy the original state. Also when the recorded and erased states are reversed, information as recorded is destroyed in reproduction similarly to the above.
In order to solve such a problem of information destruction caused in reproduction, the inventors have proposed an optical recording medium which is formed by a combination of a specific photochromic material and a polymer (Japanese Patent Application No. 3-312517 (1991)). One of the as-proposed combinations of photochromic materials and polymers is a system of the 2,3-bis(2-methylbenzo[b]thiophene-3-yl) maleic anhydride expressed in the above formula and polymethacrylate. This system has enabled reproduction of information one million times. ##STR3##
Another system is a combination of 2-(1,2-dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl) maleic anhydride and polyvinyl butyral expressed in the following formula: ##STR4##
In this system, the reproduction frequency has been improved but only to about 10.sup.2 to 10.sup.3 times.
FIG. 42 illustrates absorption spectra of the 2,3-bis(2-methylbenzo[b]thiophene-3-yl) maleic anhydride, and FIG. 43 illustrates absorption spectra of the 2-(1,2-dimethyl-3-indolyl)-3-(2,4,5-trimethyl-3-thienyl) maleic anhydride respectively. Referring to FIG. 42, the broken, solid and dotted lines show absorption spectra of the photochromic material which is in a ring-closure state, a complete ring-opening state upon irradiation with light of 546 nm in wavelength and a photostationary state upon irradiation with light of 436 nm in wavelength respectively. Referring to FIG. 43, on the other hand, the broken, solid and one-dot chain lines show absorption spectra of the photochromic material which is in a complete ring-closure state, a complete ring-opening state and a photostationary state upon irradiation with light of 436 nm in wavelength respectively.
Comparing the absorption spectra of the photochromic materials shown in FIGS. 42 and 43, it is clearly understood that the latter photochromic material is shifted to longer wavelengths by about 50 nm as compared with the former, to have sensitivity also at a wavelength of 670 nm.
Recent study of a semiconductor laser has brought development of a high output semiconductor laser in a wavelength band of 670 nm. Thus, strongly awaited is attainment of nondestructive reading with an optical recording medium employing a photochromic material having high sensitivity in such a longer wavelength band.
In the conventional photochromic compound, further, durability against repetitive recording and erasing, which is 10000 times in a deaired state as described above, is further reduced under presence of air due to strong influence by oxygen. Thus, awaited is a photochromic compound which is excellent in durability against repetitive recording and erasing under presence of air.